Microphone assembly



4Filed Jan. 11. 1954 Jan. 20, 1959 K c. D. JENKINS, JR., ET Al.2,370,255

MICROPHONE ASSEMBLY 2 Sheets-Sheet 1 41 Lgf PIE-3 2..

A Tra/WE V5 Jan. 20, 1959 c. D. JENKINS, JR., ET Al. 2,870,255

MICROPHONE ASSEMBLY 2 Sheets-Sheet 2 Filed Jan. ll. 1954 PIE A O 5 m .b

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and treated anthracite coal United States Patent 2,870,255 MICROPHONEASSEMBLY Application January 11, 1954, Serial No. 403,224

3 claims. (Cl. 179-1) This invention relates generally to microphonesused in radio, telephone and public address apparatus.

It is well known that the carbon microphone has had continuedapplication in many fields because of its simplicity and highsensitivity. The high sensitivity is due to the fact that the carbonmicrophone is actually an amplifier since the amount of electricaloutput that the diaphragm motion controls is greater than the soundpower required to operate the diaphragm. However, the carbon microphonealso has numerous disadvantages, the most important of which are thesteady background hiss resulting from random changes in the resistancewhich the carbon granules offer to the direct current .passing throughthem, the poor frequency response, and

the instability of the carbon button The carbon button is normally madeof granules made from selected and mounted against the diaphragm to forma buttonf As a result, at the present time, the carbon micro-phone isused where high sensitivity is a great asset such as in the telephonesystem, amateur, police and other radio work where a transmission ofinformation rather than good intelligibility of speech is the primaryobject, but is seldom used in a public address or high quality radiotelephone work.

lt is also well known that other types of microphones such as a variablereluctance or a dynamic microphone have frequency responsecharacteristics which are far superior to the frequency responsecharacteristics of the carbon microphone. For instance, in the dynamicmicrophone the response can be made practically fiat from 40 to 10,000cycles. However, the primary disadvantage of these other microphones isthat they have a low sensitivity in comparison with the carbonmicrophone.

In general it is the object of the present invention to provide animproved microphone which has the frequency response characteristics ofthe low sensitivity type of microphone and which has the sensitivitythat is at least as great as that of the carbon microphone.

Another object of the invention is to provide an improved device of theabove character which will fit into the same space in a handset as isrequired by a carbon microphone so that the device may be substitutedfor the carbon microphone with a minimum number of alterations in thehandset.

Another object of the invention is to provide a device of the abovecharacter which utilizes the same power supply and same voltage leadsused by the carbon microphone thereby eliminating the necessity ofadditional leads.

Additional objects and features of the invention will appear from thefollowing description in which the preferred embodiment has been setforth in detail in conjunction with the accompanying drawings.

Referring to the drawings:

Figure 1 is a view of a handset incorporating one embodiment of theinvention. The microphone assembly is shown in an exploded view.

Figure 2 is a view taken along the line 2-2 of Figure l,

and in 2,870,255 Patented Jan. 20, 1959 ICC V2 :Figure 3 is a circuitdiagram illustrating a preamplifier circuit which may be used in theinvention.

Figure 4 isv a graph comparing the audio frequency responsecharacteristics of a magnetic element, and of the magnetic elementhaving a transistor preamplifier.

Figure 5 is a graph showing the relative gain of a transistorpreamplifier with varying supply voltage.

In general the presentinvention makes use of a conventional lowsensitivity, wide frequency response, microphone element together with asub-miniature transistor preamplifier which are both placed in the samehousing to replace a carbon microphone assembly.

Referring to Figure l we have shown a handset 10 having a housing 11formed to provide a handle 12 and space for a receiving unit 13 and atransmitting unit 14. A switching unit 15 to operate the transmitter ismounted on the handle 12. A suitable conductor cable 16 leads from thehandset to the equipment with which it is desired to use the handset.

, `The body 18 of the transmitting unit 14 is a part of the variablereluctance microphone element 19 and serves as a mounting for thetransistor preamplifier 21. The

body 18 is held in place within the hollow annular por tion 17 in thehousing 11 by the retaining ring 22 which is threaded o nto the` annularportion 17. l

The microphone element 19 can be of any convention-al type having a goodfrequency responseV characteristic such as a variable reluctancemicrophone. The conductors 23 and 24 leading from the coil of themicrophone element 19 are connected to the microphone input conductors26 and 27 of the transistor of preamplifier 21. The conductors 28 and 29connect the output of the transistor preamplifier to the cable 16whereby the output of the microphone and the preamplifier are. fed intoa conventional apparatus well known to those skilled in' the radio andtelephone arts. However, any conventional means may be used forconnecting the output of the preamplifier to the cable 16 such as ringcontacts or female and male receptacles.

Any suitable audio frequency amplifier may be used for amplifying theoutput of the microphone element 19 so that the sensitivity of themicrophone element 19v will be the same or greater than that of a carbonmicrophone. lt is desirable to operate this amplifier from the samevoltage supply that was used for the carbon microphone and it is alsodesirable that the microphone and the amplifier fit within the spacepreviously occupied by the carbon microphone. By doing `this it ispossible to substitute our microphone for a carbon microphone with aminimum of effort and expense.

In View of this a preamplifier using transistors has been found to bevery desirable because of the space limitations in a conventionalmicrophone housing and the manner in which a power supply isconventionally derived. Figure 3 shows a suitable amplifier of thistype. Referring to this circuit diagram, we have shown transistors at 33and 34. Both of the transistors are of the p-n-p'diffused'alloy junctiontype. Transistor 33 is connected with base bias in a grounded emitterconfiguration whereas transistor 34 is connected with base bias in agrounded collector configuration.

As is well known in the transistor art, the transistor 33 is comprisedof a base 36, an emitter 37 and a collector 38, and the transistor 34 iscomprised of a base 39, an emitter 41 and a collector 42. The base 36 ofthe transistor 33 is connected by conductor 46 to the base bias resistor47 and the D. C. blocking and coup-ling capacitor 48. The capacitor 48is directly connected to the microphone input conductor 27. Conductor 49serves to connect the other end of base bias resistor 47 to one end ofthe collector load resistor 51 and to one end of aardgas the filtercapacitor 52 and tol one end of the voltage divider circuit comprised ofresistances S3 and S4.

Conductor 56 `is connected to the microphone input conductor 26 and isdirectly connected tothe emitter 37 of the transistor 33 throughconductor 57- and then it is connected to one end of base bias resistor58. Conductor 56 is also connected to the other end of the ltercapacitor 52 and is directly connected to the collector 42 of thetransistor 34 through conductor S9. The conductor 56 is also connectedto the midpoint between resistors -53 and 54. The collector 38 of thetransistor 33 is connected to the lother end of collector load resis tor51 by conductor 61, The conductor 61 is also connected to one side ofthe D. C. blocking and coupling capacitor 62. The other side of thecapacitor 62 is connected to the other end of base bias resistor 58 andto the'base 39 of the transistor 34. The emitter 41 of the transistor 34is'connected to the other side of the voltage divider network comprisedof resistors 53 and 54 by conductor 64. One end of the voltage dividernetwork is connected to the positive preamplifier output conductor v2Sand the other end of the voltage divider network is connected to thenegative preamplifier output conductor 29.

Suitable means is provided in the associated radio or telephoneequipment to provide a suitable source of D. C. t-o the conductors 28and 29 to energize the amplifier. Any suitable voltage from 6 volts to32 volts may be used, but in this embodiment the preamplifier has beenconstructed for an optimum of 2232 volts D. C.

The above circuit components can be mounted in any suitable manner suchas on an etched circuit board.

The circuit shown in Figure 3 operates as a conventional audio frequencyamplifier circuit using transistors. The output of the microphone isamplified a predetermined amountby the amplitier and then the output ofthe amplifier is fed into the conventional network of radio or telephoneapparatus.

By way of example, we have constructed an audio frequency amplifier asshown in Figure 3 operating .from a 22 to 32 volt D. C. sistor andhaving values of resistors and capacitors as follows: resistor 47,330,000 ohms, resistor 51, 10,000 ohms, resistor 58, 47,000 ohms,resistor 53, 820 ohms, resistor 54, 2,200 ohms, capacitor 43, 4 rnfd.,capacitor 62. .05 mid., and capacitor 52, l mfd.

This Vcircuit was found to operate very satisfactorily with a variablereluctance type of microphone to give an output which was comparable tothat of a carbon microphone with greatly improved frequency responsecharaceristics.

The graphs in Figures 4 and 5 show the characteristics of a typicalvariable reluctance microphone having a transistor preamplifierincorporated within it. Curve A of Figure 4 shows the audio frequencyresponse characteristics of a variable reluctance type of microphone byitself and curve B shows the audio frequency response characteristicsfor the same microphone having a transistor preamplifier. The particulartransistor preamplisource using CK 721 junction tranterminal to theemitter of said fier has been designed for aircraft use to give asteeply falling characteristic in the lower frequency range. This isaccomplished by using a lowvalue of capacitance for capacitor 62.

Figure 5 shows the gain characteristics of the transistor preamplifierdisclosed herein under varying supply voltages. It can be seen that thegain is nearly constant in the 22-32 volt range for which the amplifierwas designed and that it is usable at much lower voltages which might beencountered in other applications.

We claim:

l. In a microphone assembly for use as a replacement unit for a carbonmicrophone mounted in a housing having a relatively shallow circularrecess with voltage supply and outlet terminals provided in the housing,the microphone assembly comprising a body adapted to fit in said recess,a microphone element having a wide frequency response mounted on oneportion of the bottom side of'said body, a transistorized audiofrequency amplilier mounted on another portion on the bottom side ofsaid body, said microphone element and said amplifier being of a size tofit within said recess, the amplifier having input and output terminals,the input terminals being connected to the microphone element and theoutput terminals being adapted to be connected to said supply and outletterminals, and means adapted to secure the body to the housing.

2. ln a microphone assembly, first and second transistors, eachtransistor having a collector, an emitter and a base, a source of directcurrent having positive and negative terminals, circuit means connectingsaid positive first transistor, circuit means connecting said negativeterminal to the collector of said first transistor and to the emitter ofsaid second transistor, circuit means connecting the base of said firsttransistor to the collector of said second transistor, a

microphone element having a pair of output leads, circuit 'means forconnecting one of said output leads to the baseV of said secondtransistor, and necting the other output lead to the transistor.

3. In a microphone assembly, first and second transistors, eachtransistor having a collector, an emitter and a base, a source ofvoltage, circuit means for connecting said first transistor to saidsource of voltage in a grounded collector configuration with base bias,circuit means for connecting said second 'transistor to said firsttransistor and said source of voltage in a grounded emitterconfiguration with base bias, a microphone element having output leads,and means for connecting said output leadsto said second transistor andsaid source of voltage.

circuit means for conemitter of said second References Cited in the tileof this patent UNITED STATES PATENTS 2,024,705 Rutherford Dec. 17, 19352,276,933 Collom Mar. 17, 1942 2,544,027 King Mar. 6, 1951 2,579,162Veneklasen Dec. 18, 1951

